The present invention relates to the medical imaging arts. It particularly relates to the assessment by magnetic resonance imaging (MRI) of coronary disease, heart tissue viability, and cardiac function, and will be described with particular reference thereto. However, the invention will also find application in other contrast-enhanced MRI imaging of various organs, such as the kidney, brain, and liver, as well as in other imaging modalities such as computed tomography (CT).
It is well known that cardiac diseases are a leading health problem in the United States at the present time. Myocardial infarctions, popularly known as heart attacks, are a leading cause of death. However, in spite of its prevalence, diagnosis is inconsistent. More than half of the individuals who die of a heart attack do not exhibit previously recorded symptoms.
Although often undiagnosed, many catastrophic myocardial infarctions are preceded by a prolonged period of incipient coronary disease in the form of partial blockages of the smaller coronary vessels. The incipient forms of cardiac disease can produce chest pains, excessive weariness, and other symptoms which usually only manifest during periods of exercise or other strenuous exertion. These symptoms are often misdiagnosed as indigestion or other minor medical conditions. The incipient forms of coronary disease can also produce small-scale infarctions, popularly known as “silent heart attacks”, that kill portions of the heart tissue without producing clear and unambiguous symptoms.
In the early stages, the incipient forms of coronary disease are often controllable and sometimes even completely reversible, through the use of dietary and lifestyle changes or by relatively minor medical procedures such as coronary stent implantation or vascular catheterization treatments. Conversely, if left untreated the incipient forms often develop into more serious coronary diseases that lead to major heart attacks or other life threatening medical conditions. Hence, early diagnosis of incipient coronary disease is critical.
One reason why medical personnel frequently miss the early signs of incipient coronary disease is a dearth of convenient and relatively inexpensive methods for unambiguous diagnosis. The early coronary vascular blockages typically occur in small blood vessels which do not resolve well in conventional medical imaging techniques such as magnetic resonance imaging (MRI) or multi-slice computed tomography (CT). The vessels involved are too small and too numerous to practically investigate using vascular catherization diagnostic techniques. Catheterization procedures are also expensive and carry a higher degree of medical risk, further reducing their attractiveness for diagnosing ambiguous cases.
An indirect method of detecting small coronary vessel blockages is through monitoring of the uptake of an applied contrast agent bolus by heart tissue. Poorly oxygenated or dead cardiac tissue is revealed in this technique by differences in contrast agent uptake. A number of medical imaging modalities have been employed for investigating myocardial function and tissue viability, including: single-photon emission computed tomography (SPECT), typically using Th-201 or Tc-99m radiopharmaceutical agents to measure relative tissue uptake rates; positron emission tomography (PET) using an 18-F (FDG) contrast agent; and echocardiography using low dose stress agents.
MRI has also been employed for investigating myocardial function and tissue viability. In a contrast-enhanced method using a magnetic contrast agent such as a gadolinium (Gd) chelate, the Gd is administered as a bolus injection and rapid MRI measurements of several slices are performed, focusing on the ventricular or apex portions of the heart which are most commonly first affected by coronary blockages, in the hope that poorly oxygenated or dead tissues resulting from coronary vessel blockages will be detected in the Gd-sensitive MRI images due to altered Gd uptake in the damaged tissues. This approach is known as a “first-pass method” (FPM). The clinician has only a few seconds to acquire imaging data, limited by the rapid uptake of Gd into the cardiac tissue. It is critical that the area imaged during this short time includes the cardiac region affected by the vascular blockage or blockages, and a failure to do so will typically result in a failed test and potentially incorrect medical diagnosis. Thus, it is desirable to acquire image slices that substantially cover the heart volume. However, because of the limited available imaging time, tradeoffs are made between the volume coverage and the resolution with the result that full imaging coverage of the entire heart with satisfactory resolution is seldom achievable in FPM.
Once the Gd is substantially absorbed, however, it typically takes on the order of tens of minutes to hours for the Gd agent to be fully removed from the blood by the kidneys. The slow removal of Gd from heart tissue makes it difficult to reliably repeat high-quality cardiac FPM imaging using more than one Gd bolus injection in a single imaging session due to residual contamination from previous Gd boluses. This makes it preferable to acquire good images using only a single Gd bolus injection.
In the past, the MRI operator attempting to acquire FPM images for evaluating myocardial function and tissue viability has had very limited prior knowledge about appropriate imaging conditions. The operator selects several (e.g., six or seven) relatively low-resolution slices, typically axially oriented and targeting in the apex region of the heart especially including the ventricles. Due to time constraints, a more complete spatial mapping of the heart, requiring on the order of nine slices to fully cover the entire heart with adequate slice resolution, is not practical during FPM imaging. Increased speed through the use of higher gradient-slew rates may not be permissible due to safety considerations. The slice selection is made without significant foreknowledge of the defect to be imaged, and so a strong possibility exists that the selected several slices will not optimally intersect the unknown defect.
A variation on FPM are the late or delayed enhancement methods, in which imaging is performed during a time when the Gd is removed from the heart tissue. Coronary tissues with reduced or blocked blood flow typically retain the Gd longer than well oxygenated tissues, resulting in “late” contrast enhancement. Late enhancement has the advantage of a much broader acquisition time frame, on the order of minutes, compared with FPM. However, the late enhancement contrast is quite weak, and well-tuned imaging conditions are important for accurate imaging. Furthermore, the delay between administering the Gd and subsequent removal by the kidneys is variable on the order of tens of minutes to hours. Such long and uncertain delays can be problematic in a clinical environment where the MRI facility is operated on a tight schedule.
Yet another MRI method for investigating myocardial function and tissue viability employs blood oxygenation level dependence (BOLD) contrast. BOLD contrast results from the magnetic properties of the hemoglobin molecule that carries oxygen in the blood. Blood hemoglobin exists in two forms: oxyhemoglobin, which carries oxygen; and deoxyhemoglobin, which does not carry oxygen. The two hemoglobin forms have different magnetic properties: oxyhemoglobin is a diamagnetic molecule, while deoxyhemoglobin is a paramagnetic molecule. This difference in magnetic properties due to the presence or absence of oxygen is detectable by the MRI apparatus, and BOLD contrast MRI images can be produced having contrast related to the ratio of the two hemoglobin types. Since vascular blockages affect transfer of oxygenated blood into tissue and removal of deoxygenated blood from the tissue, the BOLD contrast mechanism is useful for imaging tissue oxygenation.
The BOLD contrast is typically imaged using T2* or T2 weighted imaging. BOLD contrast has been exploited in brain imaging, where it is sometimes called functional MRI, and has also been employed in cardiac imaging to a limited extent. In typical BOLD cardiac imaging, a stress agent is administered to simulate a state of high exertion which enhances oxygen demands of the cardiac tissue, followed by BOLD contrast imaging. In some cases, concern for patient safety or other issues may preclude application of the stress agent.
The present invention contemplates an improved method and apparatus for the magnetic resonance imaging (MRI) of coronary disease, tissue viability, and cardiac function which overcomes the aforementioned limitations and others.